The invention resides in an internal combustion engine for motor vehicles wherein the engine intake and exhaust valves are controlled so as to provide a certain motor braking torque depending on the vehicle operating state.
Vehicle operation without engine power may be based on two opposite objectives. In one case, the engine should generate the smallest possible motor braking torque such that the vehicle can move as far as possible without requiring engine power or it may be necessary for the engine to generate a braking torque in order to provide braking forces on downhill slopes, for example.
These objects are achieved by an internal combustion engine as described in the assignees German patent publication DE 34 28 627 A1. The engine braking torque is controllable by an exhaust gas throttle valve which is arranged in the engine exhaust duct and which causes an exhaust gas backup depending on the position of the valve. Each of the gas exhaust valves includes a limit structure which prevents the valves from fully closing. The exhaust valves are closed only to such an extent that a certain braking effect is achieved. The limit structure and the throttle valves are controllable by a brake pedal position dependent adjustable control valve.
In another procedure, DE 42 36 009 A1 discloses a method wherein the air mass flow through the internal combustion engine is controllable depending on a certain vehicle operating state and independently of a fuel volume flow in order to control the engine braking torque. During operation of the engine without power, a very small engine braking moment can be achieved when engine braking is switched off, while the throttle valve on the intake side is open. If a high engine braking torque is desired, the throttle valve and possibly also an idle air bypass are fully closed.
Furthermore, with an internal combustion engine with selectable valve opening and closing times that is for example with hydraulically or electromagnetically operated gas change valves, a very low engine braking torque can be achieved when all the gas change valves are kept constantly closed. When a high engine braking torque is desired, the gas change valves are all closed during the engine intake stroke and are shortly opened when the pistons reach their lower dead center positions in order to fill the combustion chamber with air whereupon all the valves are again closed in order to cause the air to be compressed by the piston. At the upper dead center, all the valves are then shortly opened whereby the gas is discharged and, with the valves again closed, the piston needs to generate expansion work already in the next stroke in which it moves from the top to the bottom dead center. Such procedures however generate relatively high emissions as there is a relatively high gas flow volume through the engine which furthermore may cool the catalytic converters to such a degree that they are no longer fully effective in converting the contaminants.
It is the object of the present invention to improve the emissions of such an engine and to reduce the chances that a catalytic converter is excessively cooled down during such a procedure.